The present invention relates generally to matrix materials used in making fiber reinforced composite materials, and more specifically to particulate additives to thermoplastic matrix materials to increase the modulus of the matrix materials and provide greater stabilization of the fibers.
Advanced composites are high modulus (stiffness), high strength composite materials used in applications requiring high strength to weight ratios. They are typically made by embedding very high stiffness continuous fibers, such as glass, carbon or silicon carbide, into a polymer matrix, such as an epoxy or polyester resin. The term "continuous fibers" is understood to have its commonly understood meanings in the art of composite materials, particularly that of very long fibers, generally mutually aligned for enhanced strength along a particular axis, and not chopped or very short fibers generally randomly aligned. The fibers provide stiffness and strength to the composite material, but are brittle and highly sensitive to cracks and flaws. The matrix material absorbs energy and hinders the spread of small cracks. The presence of the matrix material allows a closer approach to using the theoretical maximum strength of the fibers in practical applications.
Thermosetting resins, such as epoxies and polyesters, take a permanent set when molded, and cannot thereafter be remolded. Thermoplastic resins, however, such as nylons, polycarbonates, acetals, polyethylenes, some polyesters, polysulfone and polyetheretherketone, become soft and pliable when heated and may be remolded without changing their physical properties. This property makes thermoplastic resins, when not used as a composite material matrix, very adaptable to low cost manufacturing methods such as injection molding and extrusion.
For use in advanced composites, polymer resins are generally impregnated into continuous reinforcing fibers to form preimpregnated fiber material (generally films or sheets), called prepregs, which are laid up in laminated plies and then cured under heat and pressure to make final structural shapes. Unlike composite structures fabricated by laminating prepregs made with thermosetting resins, the overall shape of composite structures fabricated from laminated prepregs made with thermoplastic resins can, under heat and pressure, be later reshaped and reformed. Moreover, processing of the prepregs themselves is easier with thermoplastic resins than with thermosetting resins.
Unfortunately, despite their advantages of lower cost and greater flexibility and ease of use in making composite structures, composites, especially advanced composites, made with thermoplastic resins are not as strong in compression as those made with thermosetting resins. While thermoplastic resins do promise better resistance against delaminating, their lower strength in compression makes composites made with thermoplastic matrix materials generally more susceptible to fiber microbuckling, kinking and shear band formation.
Thus it is seen that there is a need for methods for increasing the compression strength of thermoplastic matrix materials without significantly affecting their desirable mechanical properties and processing and fabrication advantages.
It is, therefore, a principal object of the present invention to provide a method for increasing the compression strength of thermoplastic matrix materials used in continuous fiber reinforced composite materials.
It is a feature of the present invention that the desirable mechanical properties of the thermoplastic matrix materials are not changed and the desirable processing and fabrication conditions remain the same.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.